Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes that include a transition metal ion. According to quantum theory, an electron has a spin which can be understood as an angular momentum that produces a magnetic moment. If the electron is placed in a magnetic field the magnetic moment will tend to align with the magnetic field. However due to quantum effects, the electron can only have two states: one with the magnetic moment aligned parallel to the applied field and a second with the magnetic moment aligned anti-parallel to the field. Each of these two states has a different energy level. If electromagnetic radiation is applied at a frequency that corresponds to the separation between the two energy levels, energy is absorbed from the electromagnetic field and this absorption can be measured. An EPR spectrum can be produced by varying either the electromagnetic radiation frequency or the applied magnetic field strength and measuring the energy absorption. In practice, the latter is generally varied.
Because most stable molecules have all their electrons paired, the EPR phenomenon is not generally observable in those molecules. Some molecules, known as paramagnetic molecules, have an odd number of electrons, which obviously cannot be paired. It is these molecules that are commonly studied via EPR techniques. This limitation to paramagnetic species also means that the EPR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to EPR spectra.
In many EPR experiments, it is either advantageous or necessary to measure the EPR sample at greatly reduced temperatures (4-10K). The advantages of operating at low temperature include an increase in signal levels from samples where relaxation times are very short at room temperature and the ability to study phase transitions.
There are several methods for cooling a sample to the range of several degrees Kelvin. The most widely used method is to immerse the sample in a bath of liquid helium. However, this method has several drawbacks. Liquid helium itself is relatively expensive and, if the liquid helium must be shipped to the work site, there is inevitably some loss of liquid helium due to boil-off, making the liquid helium even more expensive. Further, as the helium evaporates, the gas is generally vented to the atmosphere and lost so that typical experiments use several liters of liquid helium each. Since helium boil-off is continuous, it is not economical to allow the EPR apparatus to remain at low temperature between experiments, thus experiments must be conducted as rapidly as possible and scheduled together to conserve helium. Finally, changing the sample temperature away from the temperature of liquid helium is difficult and can only be accomplished by changing the pressure on the helium.
In order to overcome these difficulties, systems have been developed that do not use liquid helium. These systems generally use a closed-cycle refrigerator, such as a conventional Gifford-McMahon (GM) refrigerator or a pulse tube refrigerator to cool a metal “cold head” to the required temperature. The sample to be cooled is mounted on the cold head and cooled by direct conduction. These systems also have drawbacks. First, since the sample is mechanically connected to the cold head, any vibrations produced by the refrigeration mechanism are transferred to the sample. These vibrations are typically on the order of 1-2 hertz and can cause problems with the EPR experiments. Second, in order to insulate the cold head and the sample, these latter elements are typically enclosed in a housing which is evacuated. Therefore, the cold head must be brought to a raised temperature and the housing must be vented prior to changing the sample. After the sample has been changed, the housing must be evacuated and the cold head brought down to the correct temperature, both of which are time-consuming operations.